Borrelia burgdorferi, the causative agent of Lyme disease, is maintained in nature through an infectious cycle between wild mammals and ticks. Like many bacterial pathogens, B. burgdorferi must cope with an array of changing environmental conditions to successfully persist, proliferate and be transmitted between hosts. Our broad objective is to use a molecular genetic approach to elucidate the molecular mechanisms of adaptation and variation in B. burgdorferi and their roles in the infectious cycle. B. burgdorferi resides in the midgut of an ixodid tick for a significant part of its natural life cycle. The tick vector undergoes a number of physiological and metabolic changes to which resident bacteria are exposed. Expansion of the ixodid tick integument during feeding and preparation for molting requires the synthesis of new cuticle, of which chitin, a polymer of N-acetyl glucosamine, is a component. The genome sequence of B. burgdorferi revealed several genes likely to facilitate chitin by-product utilization by the bacteria, including homologs of a transporter for chitobiose (the dimer subunit of chitin). We have begun a study to determine if these gene products facilitate chitobiose utilization and contribute to bacterial growth in ticks. We found that chitobiose efficiently substitutes for N-acetyl glucosamine during bacterial growth in culture medium. We inactivated the gene encoding the membrane - spanning component of the transporter and determined that these mutant bacteria were unable to utilize chitobiose. We also found that the transporter gene was regulated in response to growth temperature, consistent with previous results demonstrating temperature as an important environmental variable regulating differential gene expression between ticks and mammals. Initial studies on the genetics and regulation of chitobiose utilization in B. burgdorferi were conducted with an attenuated, nonpathogenic clone. We have recently inactivated the chitobiose transporter in an infectious clone. We have begun a series of experiments to compare the mutant and complemented form with its isogenic wild type parent in the natural mouse-tick infectious cycle. Such experiments have only recently become possible because of advances we have made in the genetic manipulation of pathogenic B. burgdorferi.